docs: def helical_laminar_fd_Schmidt(Re, Di, Dc):
      r'''Calculates Darcy friction factor for a fluid flowing inside a curved 
      pipe such as a helical coil under laminar conditions, using the method of 
      Schmidt [1]_ as shown in [2]_ and [3]_.
      
      .. math::
          f_{curved} = f_{\text{straight,laminar}} \left[1 + 0.14\left(\frac{D_i}
          {D_c}\right)^{0.97}Re^{\left[1 - 0.644\left(\frac{D_i}{D_c}
          \right)^{0.312}\right]}\right]
          
      Parameters
      ----------
      Re : float
          Reynolds number with `D=Di`, [-]
      Di : float
          Inner diameter of the coil, [m]
      Dc : float
          Diameter of the helix/coil measured from the center of the tube on one
          side to the center of the tube on the other side, [m]
  
      Returns
      -------
      fd : float
          Darcy friction factor for a curved pipe [-]
  
      Notes
      -----
      The range of validity of this equation is specified only for Re,
      :math:`100 < Re < Re_{critical}`.
      
      The form of the equation is such that as the curvature becomes negligible,
      straight tube result is obtained.                     
  
      Examples
      --------
      >>> helical_laminar_fd_Schmidt(250, .02, .1)
      0.47460725672835236
  
      References
      ----------
      .. [1] Schmidt, Eckehard F. "Wärmeübergang Und Druckverlust in 
         Rohrschlangen." Chemie Ingenieur Technik 39, no. 13 (July 10, 1967): 
         781-89. doi:10.1002/cite.330391302.
      .. [2] El-Genk, Mohamed S., and Timothy M. Schriener. "A Review and 
         Correlations for Convection Heat Transfer and Pressure Losses in 
         Toroidal and Helically Coiled Tubes." Heat Transfer Engineering 0, no. 0
         (June 7, 2016): 1-28. doi:10.1080/01457632.2016.1194693.
      .. [3] Pimenta, T. A., and J. B. L. M. Campos. "Friction Losses of 
         Newtonian and Non-Newtonian Fluids Flowing in Laminar Regime in a 
         Helical Coil." Experimental Thermal and Fluid Science 36 (January 2012):
         194-204. doi:10.1016/j.expthermflusci.2011.09.013.
      '''
      fd = friction_laminar(Re)
      D_ratio = Di/Dc
      return fd*(1. + 0.14*D_ratio**0.97*Re**(1. - 0.644*D_ratio**0.312))